The annealing of vacancy defects in β-NiAl II. The role of surface oxidation in vacancy loop growth in slowly cooled crystals

Abstract

Specimens of nominally stoichiometric β-NiAl have been annealed under Ar at 900°C for 24 hours, and subsequently cooled to room temperature at 5 K per hour. Annealing studies of thinned discs were performed, and observations of the change in size of vacancy dislocation loops made. In general, it was found that at temperatures above 650°C loops would shrink initially, and then grow in size with further annealing, both in a partial pressure of 10⁻⁵ Torr of O₂ and under ultra-high vacuum (< 10⁻⁸ Torr). Essentially similar results were obtained from identical studies made on specimens taken from two off-stoichiometric alloys (Ni-48·0 at.% Al and Ni-53·0 at.% Al). These results are consistent with surface oxidation acting as a source of vacancies.